XYLENE POWER LTD.

FNR INDICATOR TUBES

By Charles Rhodes, P.Eng., Ph.D.

INDICATOR TUBE:
There are 6.8 m high buoyant indicator tubes hooked on to the lifting points of movable active fuel bundles. Their positive buoyancy keeps them upright.The actual vertical position of each active movable fuel bundle is indicated by the height of the top of its indicator tube above the liquid sodium pool surface.
 

An indicator tube is a coaxial structure. The outer tube is 8.625 inch OD, 0.148 inch wall (Schedule 10). The inner tube is 1.315 inch OD, 0.109 inch wall (Schedule 10). The annular space between the two tubes is argon filled. The indicator tubes are hooked on to the lifting points of the movable fuel bundles after the movable fuel bundles have been installed.
 

INDICATOR TUBE PURPOSE:
The purposes of an indicator tube are to indicate to the overhead FNR monitoring system:
a) The actual vertical position of each movable fuel bundle in the matrix of fixed fuel bundles;
b) The gamma flux originating from each movable fuel bundle;
c) The liquid sodium discharge temperature for each movable fuel bundle.
 

INDICATOR TUBE DESCRIPTION:
Each indicator tube consists of a central 1.315 inch OD with 0.109 inch wall steel tube open at the bottom with a flat top hat containing a pin hole and a surrounding thermally isolating concentric sealed buoyant argon gas filled annular region defined by a 8.625 inch OD, 0.148 inch wall steel tube. The annular region thermally isolates the central region ensuring that the temperature of the liquid sodium in the central tube is approximately the same temperature as the liquid sodium discharge temperature of the associated movable fuel bundle.

The outer steel tube, which is thermally isolated from the inner steel tube, must withstand the external liquid sodium head pressure and the temperature dependent internal argon gas pressure. The bottom metal spacer and the top thermally insulating spacer between the inner and outer tubes keeps the annular region gas sealed.

The annular argon gas filled region, in combination with the movaable fuel bundle fuel tube plenum regions, provides a low attenuation path for gamma rays to pass vertically through the liquid sodium bath. The gammma ray flux indicates the actual fission power of each movable fuel bundle.

The central tube top cover has a small hole to vent gases trapped in the central tube. The top cover top surface must provide contrast for the temperature indicator.

The annular region of the indicator tube provides sufficient positive buoyancy so that when 1.3 m of the indicator tube is projecting above the liquid sodium surface the indicator tube maintains an upright position due to its positive buoyancy in hot liquid sodium.

Note that the buoyancy of the annular region is not sufficient to lift the weight of a movable fuel bundle when the indictor tube is fully immersed in liquid sodium.
 

THE CENTRAL TUBE TOP COVER:
The inner tube of each indicator tube assembly extends slightly above the outer tube. At the top each central tube is a 8.625 inch diameter flat thermally conductive top cover which is attached to the inner steel tube. This top cover has a small central hole that keeps the top pocket gas pressure inside the inner tube equal to the outside ambient gas pressure. This hole also allows for passaage of the temperature indicating dial shaft.

Inside the central tube is a long many turn helix of wire with a known thermal coefficient of expansion. Under a change in liquid sodium temperature the top of this helix rotates with respect to the bottom of this helix, moving a pointer that is visible from overhead which indicates the temperature of the movable fuel bundle liquid sodium discharge. The precision and reproducibility of this temperature indication is an important aspect of FNR temperature monitoring.

In operation,the liquid sodium level in the indicator tube central tube is approximately the same as the liquid sodium level outside the indicator tube.
 

LASER TARGET:
Each indicator tube presents a 8.625 inch diameter round elevation target to an overhead laser scanner. If the distance from the laser to the apparent target position is too long it means that the laser is not looking at a valid target.

Note that when the movable fuel bundles are fully inserted into the matrix of fixed fuel bundles the indicator tube top covers are about 1.3 m above the liquid sodium coolant surface.
 

GAMMA RAYS:
Gamma radiation passes up inside the fuel tube plenums, through the fuel tube top plugs, through the liquid sodium near the indicator tube attachment point, through the Indicator Tube bottom annular spacer, up the annular gas space inside the indicator tube, through the top annular thermal insulator and then through the indicator tube top hat. Note that the top annular insulator is normally out of the liquid sodium so its argon gas seal is not challenged by continuous direct exposure to hot liquid sodium. Note that the bottom annular spacer can be metal because at the bottom of the indicator tube the internal sodium and the external sodium are at the same temperature.
 

INDICATOR TUBE:
There are 6.8 m high buoyant indicator tubes field attached to the lifting points of movable active fuel bundles. The vertical position of each active movable fuel bundle is visually indicated by the 0.2 m to 1.3 m exposed height of the top of its indicator tube above the primary liquid sodium surface.
 

INDICATOR TUBE ATTACHMENT:
Indicator tubes are attached to the movable fuel bundle lifting points after the movable fuel bundles are installed and are removed before the movable fuel bundles are repositioned. The indicator tube attachment points are the movable fuel bundle lifting points. Each indicator tube has a dual J type bottom hook for attachment to the lifting points of a movable fuel bundle.
 

LIFTING POINTS:
A FNR fuel bundle lifting point is achieved by replacing the (3 / 16) inch thick diagonal plates with (3 / 8) inch thick diagonal plates in the upper portion of the fuel bundle where there are mo fuel tubes. Two 3.0 inch diameter holes in each diagonal plate form the lifting points.

The diagonal plates connecting each fuel bundle lifting point to the corresponding fuel bundle corner girders must also allow unobstructed primary liquid sodium flow and must not prevent individual fuel tube insertion or extraction.
 

INDICATOR TUBE MATERIAL AND DIMENSIONS:
The indicator tube diameter should be minimal to minimize obstruction of the natural liquid sodium circulation, but must be sufficient to allow accurate steady state movable fuel bundle liquid sodium discharge temperature measurement.

The indicator tubes are fabricated from HT-9 steel (85% Fe, 12% Cr, 1% Mo, 0% C, 0% Ni). When the movable fuel bundle is fully inserted the length of the indicator tube plus hook places the top of the indicator tube 7.4 m above the top of the movable fuel tubes. Hence the indicator tube itself is only about:
6.0 m - 0.5 m + 1.3 m = 6.8 m long.

The fuel bundle air lock inside length is long enough to accommodate indicator tubes.

Indicator Tube:
[(8.625 inch OD X 0.148 inch wall) + (1.315 inch OD X 0.109 wall)] X 6.8 m long

Indicator Tube Mass:
Mass = Pi X [(8.625 inch X 0.148 inch) + (1.315 inch X 0.109 inch] X 6.8 m X 7.874 g / cm^3
= Pi [1.2765 inch^2 + 0.14335 inch^2] X 6.8 m X 7.874 X 10^3 kg / m^3
= Pi [1.41985 inch^2] X 6.8 m X 7.874 X 10^3 kg / m^3

Displaced Volume = Pi[(4.3125 inch)^2 - (.5485 inch)^2] X 6.8 m
= Pi [18.59766 inch^2 - 0.30085 inch^2] X 6.8 m
= Pi [18.2968 inch^2] X 6.8 m

Average density = Mass / displaced volume
=[1.41985 inch^2] X 7.874 X 10^3 km /m^3 / 18.2968 inch^2
= 0.61103 X10^3 kg / m^3

When 1.3 m of the tube is projecting above the liquid sodium surface the mass stays the same but the displaced length decreases to:
6.8 m - 1.3 m = 5.5 m.
In effect the average density increases to:
0.61103 X 10^3 kg / m^3 X 6.8 m / 5.5 m = 0.7554 X 10^3 kg / m^3

The density of hot liquid sodium is:
0.84 X 10^3 kg / m^3
so that buoyancy is maintained. However, the indicator tube buoyancy is barely sufficient because in the above calculation there is no allowance for the weights of the indicator tube end spacers, the hook or the insulated top.
 

VERTICAL THERMAL EXPANSION:
Note that the open steel lattice near the bottom of the liquid sodium pool and the fuel bundles will thermally expand vertically with increasing surrounding liquid sodium temperature. During normal reactor operation the open steel lattice is likely to be about 120 degrees C cooler than the liquid sodium temperature at the top of the fuel bundle. The thermal expansion will be significant and will affect the calculation of the movable fuel bundle insertion into the matrix of fixed fuel bundles unless temperature compensating measurements are performed. Hence the overhead laser scanner may need need a compensating fixed fuel bundle elevation measurement.

The differential vertical thermal expansion per fuel bundle is approximately:
20 ppm / deg C X 430 deg C X 16.5 m = 0.1419 m
Hence it is essential that the laser scanning system cancel out vertical thermal expansion.  

MOVABLE FUEL BUNDLE TRAVEL LIMIT:
The movable fuel bundle vertical travel is limited to 1.1 m by the FNR actuator design.

The hydraulic fluid feed tubes to the FNR Actuators are routed through the open steel lattice. These hydraulic tubes must be sufficiently flexible to allow for +/- 0.5 m earthquake induced movement of the fuel assembly with respect to the primary pool walls.
 

This web page last updated May 29, 2025.